Process for electrolytically producing chlorine in a cell having a diaphragm comprising hydrophilic fluoropolymers

ABSTRACT

Certain fluoropolymers, when chemically modified by reacting them with sulfur or phosphorus containing compounds, become hydrophilic materials useful for making ion-exchange membranes, especially diaphragms for electrolytic cells, particularly chlor-alkali cells used in the production of chlorine, hydrogen and sodium hydroxide from brine.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 699,302, filedJune 24, 1976, which is a continuation-in-part of application Ser. No.579,099, filed May 20, 1975 and now abandoned.

BACKGROUND OF THE INVENTION

In commercial chlor-alkali cells used for the production of chlorine,hydrogen and sodium hydroxide from brine, asbestos diaphragms areordinarily used as anion exchange membranes between the anolyte andcatholyte compartments.

Diaphragms of this sort are generally satisfactory, but their electricalresistance is high and chlor-alkali cells which use such diaphragmstherefore require more electric current for operation than is desirable.

SUMMARY OF THE INVENTION

It has now been found that by chemically modifying certainfluoropolymers with sulfur or phosphorus containing compounds, thefluoropolymers are made hydrophilic. This makes them especially suitedfor use in making diaphragms for electrolytic cells.

Such diaphragms have less electrical resistance than those made solelyof asbestos. There is, accordingly, less power loss in a chlor-alkalicell which uses this new diaphragm and the cell is therefore moreefficient. Indeed, it has been observed that the chlorine, hydrogen andcaustic production of a cell using this diaphragm can generally be heldat the same level as one using an asbestos diaphragm while operating at10-15% lower voltage.

The hydrophilic fluoropolymers also have the inertness of fluoropolymersgenerally, and this makes them resistant to attack by the contents ofconventional electrolytic cells.

"Hydrophilic", as used in this context, means that the modifiedfluoropolymers do not repel water as conventional fluoropolymers do, butinstead absorb it. The term more specifically means that the modifiedfluoropolymers have water contact angles of 0° to about 50°, as measuredby the method and apparatus described on page 137 of "Contact Angle,Wettability and Adhesion", American Chemical Society, 1964.

Specifically, the hydrophilic fluoropolymers of the invention arechemically modified homopolymers, or copolymers (meaning they containtwo or more different monomer units), derived at least in part fromolefinic monomers completely substituted with fluorine atoms orcompletely substituted with a combination of fluorine atoms and no morethan one chlorine, bromine or iodine atom per monomer. Thesefluoropolymers can also contain up to about 75 mol percent of unitsderived from other ethylenically unsaturated monomers.

These hydrophilic fluoropolymers contain non-terminal units representedby the structure ##STR1## where M is hydrogen, sodium, potassium,lithium, calcium or magnesium;

R is an alkyl radical of 1-12 carbon atoms or a cycloalkyl radical of3-12 carbon atoms; and

x is 1, 2, 3, 4 or 5.

Hydrophilic fluoropolymers containing non-terminal units of structure(1) where Z is ##STR2## are preferred for use as diaphragm materials inelectrolytic cells because of their low electrical resistance.

The hydrophilic fluoropolymers can contain more than one type of unit ofstructure (1).

The hydrophilic fluoropolymer will contain enough units of structure (1)to give it a sulfur or phosphorus content of about 0.1%-10%, by weight,but not so many that more than about 1%, by weight, of the polymer willdissolve in water at 20° C.

Sulfur and phosphorus content is determined gravimetrically by

(1) converting the sulfur present to a sulfate in an oxygen flask, thenadding BaCl₂ and weighing the BaSO₄ precipitate, as described by J. H.Karchmer on pages 112-114 of "The Analytical Chemistry on Sulfur and itsCompounds, Part I", Wiley-Interscience, 1970; or

(2) converting the phosphorus present to a phosphate by the Schonigeroxygen flask method, then adding ammonium molybdate and weighing theammonium molybdophosphate precipitate, as described by M. Halmann onpages 15, 16, 22 and 23 of "Analytical Chemistry of PhosPhorusCompounds", Wiley Interscience, 1972.

Water solubility is determined by running a sample of polymer in aSoxhlet extractor for 24 hours, using deionized water as a solvent.

When a hydrophilic fluoropolymer of the invention is to be used as adiaphragm material in an electrolytic cell, it preferably containsenough units of structure (1) to give it an electrical resistance ofabout 0.1-100 ohms per square centimeter of surface area. Resistance isdetermined using conventional instruments to measure the voltage andamperage of a current flowing through the material under actual useconditions, computing the resistance using Ohm's law, and then dividingthe resistance by the area, in square centimeters, of the material.

The preparative methods to be described will give hydrophilicfluoropolymers with the foregoing chemical and electricalcharacteristics.

The hydrophilic fluoropolymers of the invention containing units ofstructure (1) which are ionic are useful as materials for ion-exchangemembranes. The hydrophilic fluoropolymers which contain units ofstructure (1) which are nonionic can be used as materials forsemi-permeable membranes in osmotic procedures and in dialysis.

DETAILED DESCRIPTION OF THE INVENTION How The Hydrophilic FluoropolymersAre Made

The hydrophilic fluoropolymers of the invention can be made by twomethods. Each method requires a fluoropolymer starting material and amodifying compound.

"Fluoropolymer starting material", as used here, means a homopolymer orcopolymer (meaning the copolymer contains two or more different monomerunits) derived at least in part from olefinic monomers completelysubstituted with fluorine atoms or completely substituted with acombination of fluorine atoms and no more than one chlorine, bromine oriodine atom per monomer.

Representative of such fluoropolymer starting materials are homopolymersand copolymers (in all monomer unit weight ratios) of

(1) tetrafluoroethylene (TFE)

(2) hexafluoropropylene (HFP)

(3) chlorotrifluoroethylene (CTFE) and

(4) bromotrifluoroethylene. (BTFE)

These fluoropolymer starting materials can also contain up to about 75mol percent of units of other ethylenically unsaturated monomers whichcontain at least as many fluorine atoms as carbon atoms, for example,vinylidene fluoride. Optionally, minor amounts of olefins containing 2-4carbon atoms can also be present. Illustrative of the fluoropolymerstarting materials which result from use of these other ethylenicallyunsaturated monomers are those described on pages 9 and 10 of copendingapplication Ser. No. 562,648, filed Mar. 27, 1975 by Apotheker andKrusic.

When these other ethylenically unsaturated monomer units are present ina fluoropolymer starting material which is to be modified and made intoa diaphragm for an electrolytic cell, it is preferred that the units ofstructure (1) be present in the form ##STR3## to avoid possiblealpha,beta elimination of hydrogen fluoride under extreme conditions ofheat and alkalinity found in some electrolytic cells.

The fluoropolymers preferred as starting materials because of the lowelectrical resistance of the hydrophilic fluoropolymers which resultwhen they are used are

(1) polytetrafluoroethylene (PTFE)

(2) polychlorotrifluoroethylene (PCTFE)

(3) polybromotrifluoroethylene (PBTFE) and

(4) copolymers (in all monomer unit weight ratios) of

(a) TFE and HFP

(b) TFE and CTFE

(c) TFE and BTFE

(d) CTFE and BTFE and

(e) TFE and perfluoroalkyl vinyl ether, (especially perfluoromethylvinyl ether).

PCTFE, the TFE-BTFE copolymers and the TFE-CTFE copolymers areespecially preferred because their reactivity makes it easier to preparethe hydrophilic fluoropolymers of the invention from them.

Mixtures of fluoropolymer starting materials can also be used.

The molecular weight of the fluoropolymer starting material, and of thehydrophilic fluoropolymer product, is unimportant. A low molecularweight fluoropolymer which itself may lack the physical strength neededfor the use intended can be coated on a fabric substrate made of suchthings as glass fibers, asbestos or PTFE fibers and then modified, orcan be crosslinked after modification, to provide the strength.

The fluoropolymer starting materials are either commercially availableor can be made by well known methods. See for example, "Fluoropolymers",Leo A. Wall, Wiley-Interscience, 1972, Chapter 1, "Polymerization ofFluoroolefins", pages 1-29.

In both methods of preparing the hydrophilic fluoropolymer, thefluoropolymer starting material can be in the form of a powder, a porousor nonporous unsupported film, a porous reinforced structure, a coatingon an inert fabric, or in the form of fibers.

The fluoropolymer starting materials are available commercially, or canbe conventionally made, as powders or as dispersions from which powderscan be extracted. These powders ordinarily contain particles having anaverage longest dimension of about 0.1-10 microns, with no particleshaving a longest dimension greater than about 50 microns. Thesedimensions are measured optically, against a standard.

A porous unsupported film can be made from powders of fluoropolymerstarting material according to conventional cellulose spongemakingprocedures. Sodium chloride or calcium chloride (about 50-60%, by weightof the fluoropolymer starting material) is first dissolved in an aqueousdispersion of fluoropolymer starting material (about 20%-25% by weightof solids). This solution-dispersion is then cast on a plate to athickness of about 0.1-5 millimeters, air-dried and then heated to fusethe fluoropolymer starting material. The resulting film is cooled,stripped from the plate and then immersed in water for 2-3 days at 20°C. to extract the chloride and give a porous unsupported film offluoropolymer starting material.

A nonporous unsupported film of fluoropolymer starting material can bemade according to the foregoing procedure by simply omitting theaddition of sodium chloride or calcium chloride to the polymerdispersion, and omitting the subsequent immersion in water.

A porous reinforced structure of fluoropolymer starting material can bemade by coating a fabric of glass fibers, asbestos, PTFE fibers, or thelike, with a dispersion containing about 5%-25% by weight of afluoropolymer starting material and about 10%-25% by weight of a polymersoluble in the carrier and stable at the fluoropolymer fusiontemperature, such as polyvinyl acetate or an acrylic polymer such aspolymethylmethacrylate. The carrier for the dispersion is a solvent forthe polymethylmethacrylate, such as toluene or methylisobutyl ketone. Ifthe fluoropolymer starting material is other than a TFE-HFP copolymer,adhesion of that polymer to the fabric can be improved by adding about2%-5%, by weight of the dispersion, of TFE-HFP copolymer (85/15 weightratio) to the dispersion. The dispersion is applied to both sides of thefabric to give a final structure thickness of about 0.1-5 millimeters.

The coated fabric is heated to fuse the fluoropolymer starting materialand give a continuous coating. The fabric is then soaked at 50°-130° C.for 2-4 hours, with agitation, in a solvent for thepolymethylmethacrylate, such as dimethylacetamide or acetone. Thisleaches out the polymethylmethacrylate, giving a porous reinforcedstructure of fluoropolymer starting material.

A nonporous coated fabric can be made by coating a fabric of glassfibers, PTFE fibers, asbestos, or the like, with a dispersion offluoropolymer starting material, drying it and then heating it to fusethe fluoropolymer. Enough of the dispersion should be used to give afinal coating weight of about 0.001-0.5 gram of polymer per squarecentimeter of fabric and a final coated fabric thickness of about 0.1-5millimeters, as measured with a micrometer. A porous coated fabric canbe made in the same manner by limiting the final coating weight of thefluoropolymer starting material to about 0.01-0.10 grams of polymer persquare centimeter of fabric.

Fibers of fluoropolymer starting material can be made by mixingfluoropolymer particles with cellulose xanthate, forcing the mixturethrough a suitable spinnerette and then baking the resulting fibers atthe fluoropolymer's fusion temperature to drive off the xanthate. Thisprocedure is described in Burrows and Jordan, U.S. Pat. No. 2,772,444,granted Dec. 4, 1956. These fibers ordinarily have an average diameterof about 1-20 microns, with no fibers having a diameter greater thanabout 50 microns. These diameters are measured optically, against astandard.

In an alternative and preferred procedure, a porous reinforced structurecan be made from a dispersion which comprises

(a) a fluoropolymer starting material;

(b) a fibrous material which will act as a base for the structure;

(c) optionally, a fluoropolymer binder material; and

(d) a liquid carrier.

This composition can also contain conventional adjuncts such as wettingagents, surfactants, defoamers and the like, in the usual amounts.

A porous reinforced structure can be made from such a composition byfirst deagglomerating the fibers of (b) and then forming a mat of thefibers by removing the carrier, preferably by a papermaking technique.In a highly preferred embodiment of the invention, this porousreinforced structure is formed directly on the cathode screen of anelectrolytic cell.

Any fibrous material can be used in (b) which can withstand the bakingtemperature to be used and which resists attack by the environment inwhich the structure is to be used. Illustrative of such materials are

asbestos

glass fibers

fibers of such fluoropolymers as

Ptfe or TFE-HFP copolymers and

potassium titanate fibers.

Mixtures of such fibrous materials can also be used. Asbestos is thepreferred fibrous material for use in electrolytic cell diaphragms.Especially preferred is a chrysotile asbestos whose fibers have anaverage diameter of about 200 A (as measured by electron microscopy) andan average length of about 70 mm.

Similarly, the fluoropolymer to be used as a binder material in (c) canbe any which resists attack by the environment in which it is to beused. Illustrative are

Ptfe

tfe-hfp copolymers (all monomer ratios)

polyvinyl fluoride

polyvinylidene fluoride and

vinylidene fluoride/hexafluoropropylene copolymers (all monomer ratios).

Mixtures of binder materials can also be used.

In electrolytic cell diaphragms, the TFE-HFP copolymers are preferred asbinder materials because of their inert nature.

The carrier in (d) can be any liquid which will not significantly affectthe chemical or physical characteristics of the structure. Illustrativeof such liquids are

water

chlorinated hydrocarbons

methanol

hexane and

brine

When the composition is to be used to make an electrolytic celldiaphragm, a 15% by weight brine solution is preferred as a carrierbecause it helps keep the fibrous material in suspension.

The components of the composition are preferably present in thefollowing concentrations:

(a) fluoropolymer starting material--10-90% by weight of the total of(a) and (b), even more preferably 40-60%;

(b) binder--10-90% by weight of the total of (a) and (b), even morepreferably 40-60%;

(a) plus (b) constituting 10-90%, by weight of the total of (a), (b),and (c), preferably 20-25%;

(c) fibrous material--10-90%, by weight of the total of (a), (b), (c),even more preferably 75-80%; and

(d) carrier--the remainder.

The composition will contain 0.01-3%, by weight, of solids, preferablyabout 1%.

PREPARATIVE METHOD ONE

In the first method of preparation, the fluoropolymer starting material,in whatever form, is exposed to radiation while it is in contact with asuitable modifying compound, or the fluoropolymer and modifying compoundare brought together in the presence of a peroxy compound such asbenzoyl peroxide of t. butyl peroxide.

Any compound which contains a sulfur or phosphorus atom which can reactwith the fluoropolymer starting material can serve as a modifyingcompound. Representative of these are ##STR4## where M is hydrogen,sodium, potassium, lithium, calcium or magnesium;

R is an alkyl radical of 1-12 carbon atoms or a cycloalkyl radical of3-12 carbon atoms; and

x is 1, 2, 3, 4 or 5.

Mixtures of modifying compounds can also be used.

The irradiation is accomplished by first placing a fluoropolymerstarting material, in whatever form, in a saturated solution ofmodifying compound in water or other inert solvent such as benzene. Ifthe modifying compound is itself a liquid, no other liquid need be used.The amount of solution or liquid in which the fluoropolymer startingmaterial is placed should be such that the starting material isthoroughly wetted, but not so much that the passage of radiation throughthe liquid is impeded. About 0.1-1%, by weight, of a surfactant such asperfluorooctanoic acid may be added to aid wetting.

The liquid containing the fluoropolymer starting material and modifyingcompound is then exposed to about 0.1-20 megarads of electron radiationor to about 0.1-50 watts per square centimeter of liquid surface ofultraviolet light, at 20° C.

The irradiation is done in conventional equipment. If desired, it can beperformed continuously, with fluoropolymer starting material andmodifying compound being fed into the equipment and hydrophilicfluoropolymer of the invention being removed.

After the irradiation step is completed, the product is separated fromthe liquid and washed with water. Drying is optional and can be doneconventionally.

The irradiation also crosslinks the hydrophilic fluoropolymers of theinvention based on PCTFE and TFE-CTFE and TFE-BTFE copolymer startingmaterials, thereby physically strengthening them.

When the fluoropolymer starting material and modifying compound arebrought together in the presence of a peroxy compound, the procedure isthe same except that irradiation is omitted. Peroxy compound, about10-100 mol percent of the fluoropolymer starting material, is added tothe liquid in which the fluoropolymer has been placed. This liquid, withthe fluoropolymer in it, is then held at 80°-150° for 5-36 hours. Thepolymer is then separated from the liquid, washed with water, and,optionally, dried.

PREPARATIVE METHOD TWO

In this method, the fluoropolymer starting material and modifyingcompound are brought into contact with each other, under conditionssuitable for reaction, and kept together until the reaction is complete.

This method can only be used with homopolymers and copolymers of CTFEand BTFE as starting materials.

The modifying compounds used are ##STR5## where M is hydrogen, sodium,potassium, lithium, calcium or magnesium;

R is an alkyl radical of 1-12 carbon atoms or a cycloalkyl radical of3-12 carbon atoms; and

x is 1, 2, 3, 4 or 5.

Mixtures of these compounds can also be used.

The fluoropolymer starting material, preferably in the form of a porousunsupported film or a porous reinforced structure, is placed in a2%-10%, by weight, solution of modifying compound in dimethylformamideor dimethylacetamide. As in Preparative Method One, only enough of thesolution is used to thoroughly wet the fluorocarbon polymer startingmaterial. If desired, a small amount of cesium fluoride or potassiumfluoride can be added to the solution to suppress side reactions.

The solution, with the fluoropolymer starting material in it, is thenbrought to a temperature within the range of about 20° C. to about theboiling point of the solvent used, to bring about the reaction. Thesolution is held at that temperature for about 4-200 hours, the reactionat lower temperatures requiring more time for completion than onecarried out at higher temperatures. When the initially colorlessfluoropolymer starting material turns black, the reaction is complete.

In both methods of preparation, the modifying compounds used willprovide radicals which will be attached directly to the fluoropolymerchain. These compounds, and the radicals they provide, are shown in thefollowing table:

    ______________________________________                                                            Attached                                                  Compound            Radical (Z Group)                                         ______________________________________                                        Sulfur              S.sub.x Br or S.sub.x Cl                                  S.sub.2 Cl.sub.2    SCl                                                       SOCl.sub.2          SOCl                                                      SO.sub.2 Cl.sub.2   SO.sub.2 Cl                                               M.sub.2 S.sub.x     S.sub.x M                                                  ##STR6##                                                                                          ##STR7##                                                  ##STR8##                                                                                          ##STR9##                                                  ##STR10##                                                                                         ##STR11##                                                 ##STR12##                                                                                         ##STR13##                                                 ##STR14##                                                                                         ##STR15##                                                 ##STR16##                                                                                         ##STR17##                                                 ##STR18##                                                                                         ##STR19##                                                 ##STR20##                                                                                         ##STR21##                                                PCl.sub.3           PCl.sub.2                                                 POCl.sub.3          POCl.sub.2                                                 ##STR22##                                                                                         ##STR23##                                                 ##STR24##                                                                                         ##STR25##                                            

where

M is hydrogen, sodium, potassium, lithium, calcium or magnesium;

R is an alkyl radical of 1-12 carbon atoms or a cycloalkyl radical of3-12 carbon atoms; and

x is 1, 2, 3, 4 or 5.

It should be noted that the radicals which result when the sulfurcompounds in the following list are used as modifiers can be convertedinto --SO₃ H radicals by treating the hydrophilic fluoropolymers bearingthese radicals with saturated chlorine water or 25%-50% (by weight)aqueous hydrogen peroxide for 2-20 hours at 20° C. The radicals whichresult when the phosphorus compounds are used can be converted into##STR26## radicals by treating the hydrophilic fluoropolymers bearingthese radicals with a 5% solution of NaOH in methanol. ##STR27## where Mis hydrogen, sodium, potassium, lithium, calcium or magnesium;

R is an alkyl radical of 1-12 carbon atoms or a cycloalkyl radical of3-12 carbon atoms; and

x is 1, 2, 3, 4 or 5.

HOW THE HYDROPHILIC FLUOROPOLYMERS ARE PUT INTO USABLE FORM

Unmodified porous and nonporous unsupported films and unmodified porousreinforced structures of fluoropolymer starting material, and unmodifiednonporous fabrics coated with fluoropolymer starting material, preparedby the methods previously described and then modified by PreparativeMethod One or Preparative Method Two, can be used directly withoutfurther fabrication.

Felt can be prepared from fibers of hydrophilic fluoropolymer usingwell-known felting methods.

A porous unsupported film can be made from a powder of hydrophilicfluoropolymer according to the conventional spongemaking techniquesalready described.

When these porous products are to be used as diaphragms in electrolyticcells, they preferably should be able to pass about 0.01-5 cubiccentimeters of liquid per square centimeter of surface area per minute.This flow rate is measured by conducting a predetermined amount ofliquid through the product, collecting the liquid and measuring theamount collected, measuring the time required to collect the liquid,dividing the amount collected by the time required to collect it, andthen dividing that quotient by the area of the product (in squarecentimeters).

The flow rate requirement will vary with the use. When used inchlor-alkali cells, for example, porous unsupported films or porousreinforced structures should be able to pass about 0.02-1 cubiccentimeters of brine per square centimeter of surface area per minute.

In a porous unsupported film or a porous reinforced structure, theproper flow rate can be obtained by varying the thickness of the film orstructure, or by varying the porosity through changing the ratio ofchloride or polymethylmethacrylate to fluoropolymer starting material orhydrophilic fluoropolymer in the fabrication step. Varying the chlorideratio is done routinely in the cellulose sponge art according towell-known principles, and the desired porosity can be obtained in afilm or a reinforced structure by applying those principles to thistechnology.

Ordinarily, the proper flow rate can be obtained with pores having anaverage longest transverse (to the direction of flow) dimension of about0.1-10 microns, with no transverse dimension larger than about 15microns. Pore size is measured optically, against a standard. The porousunsupported film or porous reinforced structure will have a pore densityof about 10,000-1,000,000 pores per square centimeter of surface area,as counted with the aid of a microscope.

If the porous unsupported film or porous reinforced structure is to beused as a diaphragm material for a chloralkali cell, its poresordinarily have an average longest transverse (to the direction of flow)dimension of about 0.1-2 microns, with no transverse dimension largerthan about 10 microns, and a pore density about 10,000-1,000,000 poresper square centimeter.

If these porosities and pore densities do not give precisely the flowrate desired, a few simple measurements followed by appropriateadjustments in the amount of chlorides or polymethylmethacrylate usedshould give the proper flow rate. In the case of felt, the proper flowrate can be obtained by coordinating its thickness with the number offibers per unit volume, as is well known.

The tensile strengths of the various products just described need onlybe high enough to enable the forms to withstand the stresses encounteredin use. These products have the requisite strength inherently.

After a porous or nonporous unsupported film, porous reinforcedstructure, nonporous coated fabric or porous felt of hydrophilicfluoropolymer has been made, it can be used directly for whatever use isintended by simply trimming it to the correct dimensions and placing itin position in the apparatus used.

The preferred method of preparing a diaphragm for a chloralkali cell isthe same as that previously described for making a preferred porousreinforced structure starting material, except that a finishedhydrophilic fluoropolymer is used instead of a fluoropolymer startingmaterial. As in the previously described method, a diaphragm can be madefrom a dispersion which comprises

(a) a hydrophilic fluoropolymer, preferably in the form of a powder;

(b) a fibrous material which will act as a base for the diaphragm;

(c) optionally, a fluoropolymer binder material; and

(d) a liquid carrier.

This composition, as the one previously described, can also containconventional adjuncts such as wetting agents, surfactants, defoamers andthe like, in the usual amounts.

A chlor-alkali cell diaphragm can be made from such a composition byfirst deagglomerating the fibers of (b) and then forming a mat of fibersby removing the carrier, preferably by a papermaking technique. Evenmore preferably, the diaphragm is thus formed in situ on the cathodescreen of the cell. However formed, the diaphragm is then heated to fusethe fluoropolymer binder, if it is present. The diaphragm is then readyfor use.

The fibrous materials, the fluoropolymer binders, the carriers, and thepreferred embodiments and concentrations of these are as previously setforth in the description of preparing a porous reinforced structure formodification according to the invention.

Diaphragms for electrolytic cells, made in situ in this way and aspreviously described, must meet the operator's specifications regardingpermeability, current efficiency and dimensional stability. Thesespecifications vary with the operator, the type of cell being used,electrical current demands of the cell, and like factors. One skilled inthe diaphragm making art will use the same skills in preparing these insitu diaphragms that he does in preparing conventional asbestosdiaphragms, and by applying those skills will, without difficulty, beable to prepare diaphragms that will meet all of any manufacturer'srequirements.

While the hydrophilic fluoropolymers of the invention are most useful asdiaphragm materials for electrolytic cells, especially chlor-alkalicells, they can also be used as materials for membranes to be used inother ion-exchange procedures, as in desalinization of sea water, and asmaterials for semi-permeable membranes to be used in osmotic proceduresand in dialysis.

When a hydrophilic fluoropolymer is to be used as a membrane for osmosisor dialysis, an appropriate modifying compound must be used to make thepolymer properly ionic or non-ionic, as is well known in the art.

If contaminants which are present in most electrolytic cells,ion-exchange cells or dialysis chambers clog a diaphragm or membrane ofhydrophilic fluoropolymer to the point where its efficiency isdiminished, the diaphragm or membrane can be rejuvenated byback-flushing it with water, rinsing it with concentrated nitric acidand then with water.

The following examples illustrate the invention. In these examples, allparts and percentages are by weight, unless otherwise indicated.

EXAMPLE 1

A piece of "Teflon" fluorocarbon resin cloth (T-162-42, sold by Stern &Stern Textiles, Inc.) was preshrunk by baking it for 10 minutes at 270°C.

A 15% solids aqueous dispersion oftetrafluoroethylene/bromotrifluoroethylene 86/14 copolymer was sprayedon both sides of the cloth to an overall thickness of 100 microns (dry)and the cloth was then baked for 20 minutes at 270° C.

The resulting coated fabric was then placed in a vessel containing asolution of

    ______________________________________                                        Potassium sulfide   11 parts                                                  Sulfur             6.4 parts                                                  Cesium fluoride    3.0 parts                                                  ______________________________________                                    

in 300 parts of dimethylacetamide.

The vessel was heated on a steam bath for four hours, while the solutioncontaining the fabric was stirred.

The coated fabric was then removed from the vessel, washed withdistilled water, submerged in saturated chlorine water and kept therefor 16 hours at 20° C.

The fabric was then rinsed with distilled water, dried and placed in thediaphragm position of a laboratory chlor-alkali cell containingsaturated brine, where, in operation, it required a voltage of 3.0-3.1to achieve a current density of 0.204 amperes per square centimeter ofdiaphragm area.

The flow rate through the diaphragm was found to have been 1.87 cubiccentimeters of brine per square centimeter per minute.

The fabric coating had a water solubility of less than 1%.

EXAMPLE 2

    ______________________________________                                        (A)   The following were added to a vessel:                                         Dispersion of polychlorotri-                                                                           200 parts                                            fluoroethylene sold by 3M Co.                                                 as "Kel-F" (30% in methyl-                                                    isobutyl ketone)                                                              Dispersion of tetrafluoroethylene/                                                                      50 parts                                            hexafluoropropylene 85/15 co-                                                 polymer resin (30% in methyl-                                                 isobutyl ketone)                                                              Solution of polymethylmethacry-                                                                        188 parts                                            late resin MW.sub.n 65,000-100,000                                            (40% in acetone/toluene 2/1                                                   by volume)                                                              ______________________________________                                    

These components were well mixed and a piece of "Teflon" cloth (as perExample 1) dipped into the mixture. The coated cloth was removed, excessmixture was removed by drawing it down with wire-wound rods, and thecloth baked for 30 minutes at 270° C.

The coated cloth was cooled to room temperature and placed in a vesselcontaining a mixture of xylene and dimethylacetamide (1/1 by volume).The temperature of the mixture was raised to the boiling point of theliquid over a two-hour period and held there for two hours. Theresulting porous reinforced structure was then removed.

(B) The porous reinforced structure made in (A) was rolled into acylindrical shape and placed in a vessel containing a solution of

    ______________________________________                                        Potassium sulfide  45 parts                                                   Sulfur              5 parts                                                   Cesium fluoride     2 parts                                                   ______________________________________                                    

in 1500 parts of dimethylacetamide. The vessel was sealed and rolled for88 hours. The porous structure was then removed, washed with distilledwater, submerged in saturated chlorine water and kept there for threehours at 20° C., with stirring.

The porous reinforced structure was then dried and placed in thediaphragm position of a laboratory chloralkali cell containing saturatedbrine, where, in operation, it required an average voltage of 3.06 toachieve a current density of 0.204 amperes per square centimeter ofdiaphragm area.

The flow rate through the diaphragm was found to have been 1.04 cubiccentimeters of brine per square centimeter per minute.

The cloth coating had a water solubility of less than 1%.

EXAMPLE 3

A porous reinforced structure was prepared as in Example 2(A).

This was air-dried, thoroughly soaked with dimethylphosphite and thengiven one megarad of electron radiation with an electron beam resonanttransformer.

The irradiated porous structure was placed in a 5% solution of sodiumhydroxide in methanol, which was then heated to reflux temperature andheld there for one hour, with stirring. The structure was removed,washed in distilled water for 15 minutes, dried and placed in thediaphragm position of a laboratory chlor-alkali cell containingsaturated brine.

Direct current was applied. The current flow initially was one ampere at8.8 volts; after two hours, the flow was one ampere at 8.1 volts. Theflow rate was found to have been 0.074 cubic centimeters of brine persquare centimeter per minute.

The cloth coating had a water solubility of less than 1%.

EXAMPLE 4

    ______________________________________                                        (1)   A mixture of                                                                  Tetrafluoroethylene/bromo-                                                                             22 parts                                             trifluoroethylene 84/16                                                       copolymer powder                                                              Methylisobutyl ketone    66 parts                                       was ball-milled for 30 minutes.                                               (2)   The dispersion of (1) was added to a mixture                            Tetrafluoroethylene/hexa-                                                                              18.3 parts                                           fluoropropylene 85/15                                                         copolymer resin dispersion                                                    (30% in methylisobutyl ketone)                                                Solution of polymethylmeth-                                                                            68.8 parts                                           acrylate resin MW.sub.n 65,000-                                               100,000 (40% in acetone-                                                      toluene 2/1 by volume)                                                        Methylisobutyl ketone      60 parts                                           Aminopropyltrimethoxysilane                                                                              1 part                                             ______________________________________                                    

(3) A piece of asbestos paper of the type used in chlor-alkali cells wasthoroughly soaked in the product of (2) and then baked for 30 minutes at315° C.

(4) The porous reinforced structure of (3) was placed in a vesselcontaining a solution of

    ______________________________________                                        Dimethylacetamide    850 parts                                                Potassium polysulfide                                                                               33 parts                                                Potassium fluoride    13 parts                                                ______________________________________                                    

The vessel was filled with nitrogen, sealed and held for 8 days at roomtemperature. The coated paper was then removed.

(5) The product of (4) was soaked in saturated chlorine water for 8hours and then air-dried.

The resulting material was suitable for use as a diaphragm material in achlor-alkali cell.

EXAMPLE 5

    ______________________________________                                        (1)   A mixture of                                                                  Tetrafluoroethylene/chloro-                                                                          16.5 parts                                             trifluoroethylene 87.5/                                                       12.5 copolymer powder                                                         Methylisobutyl ketone    50 parts                                       was ball-milled for 30 minutes.                                               (2)   To the dispersion of (1) was added a mixture of                         Tetrafluoroethylene/hexa-                                                                              13.7 parts                                           fluoropropylene 85/15                                                         copolymer resin dispersion                                                    (30% in methylisobutyl                                                        ketone)                                                                       Solution of polymethylmeth-                                                                            51.6 parts                                           acrylate resin MW.sub.n 65,000-                                               100,000 (40% in acetone/                                                      toluene 2/1 by volume) -Aminopropyltrimethoxysilane                                                    0.75 part                                            ______________________________________                                    

(3) The ball mill used in (1) was rinsed with 15 parts of methylisobutylketone, which was added to the dispersion of (2). An additional 22.5parts of methylisobutyl ketone was then added to the dispersion of (2).

(4) A piece of asbestos paper of the type used in chlor-alkali cells wasthoroughly soaked in the product of (3) and air dried. Twenty parts ofmethylisobutyl ketone was added to (3) and the paper again thoroughlysoaked in it. The paper was then air-dried and baked for 30 minutes at315° C.

(5) The following were placed in a glass jar:

    ______________________________________                                        Dimethylacetamide    400 parts                                                 (dried over molecular                                                         sieves)                                                                      Potassium sulfide     2.2 parts                                               Sulfur                6.4 parts                                               ______________________________________                                    

The jar was sealed and rolled for two hours at room temperature.

(6) The liquid product of (5) was placed in a vessel. The porousreinforced structure of (4) was submerged in the liquid and the vesselplaced in an enclosure. The enclosure was purged with nitrogen, sealedand allowed to stand undisturbed for two weeks at room temperature.

The resulting material was suitable for use as a diaphragm material in achlor-alkali cell.

EXAMPLE 6

A hydrophilic fluoropolymer of the present invention was prepared by

(a) providing a fluoroelastomer which is a copolymer containing (on apercent by weight basis) 58.0% vinylidene fluoride units, 39.1%hexafluoropropylene units and 2.9% bromotrifluoroethylene units, saidfluoroelastomer having a Mooney viscosity of 38 at 100° C. when measuredon a Mooney viscometer using the large rotor and a ten-minute shearingtime;

(b) grinding said fluoroelastomer into small particles by using a freezemill in which the fluoroelastomer was frozen with liquid nitrogen andthen hammer-shattered into small particles;

(c) mixing 20 grams of the resulting particulate fluoroelastomer with 20grams of dimethylphosphite;

(d) mixing one gram of benzoyl peroxide with the resulting mixture;

(e) heating the resulting mixture at reflux (172° C.) for 3 hours;

(f) mixing one gram more of benzoyl peroxide with the resulting mixture;

(g) heating the resulting mixture at reflux for 2 hours, and thencooling it to about 25° C. and pouring it into one liter of water at 25°C.;

(h) washing the modified fluoroelastomer present in the resultingmixture five times with fresh distilled water, each washing cycleincluding depositing the fluoroelastomer on a filter;

(i) mixing the fluoroelastomer with one liter of acetone at 22° C.;

(j) slowly adding the resulting mixture to water at 22° C. with stirringto precipitate the fluoroelastomer, followed by filtration; and

(k) drying the resulting modified fluoroelastomer in a vacuum over fortwo days at 60° C.

The fluoroelastomer described in step (a) was made by

(1) continuously feeding (per hour) 56 parts of vinylidene fluoride, 44parts of hexafluoropropylene and 2.8 parts of bromotrifluoroethylene toa two-liter stainless steel pressure vessel reactor which had beenflushed with nitrogen while operating the stirrer of the reactor at 500rpm for thorough mixing of the reactor contents, and while the contentsof the reactor were heated at 105° C. under a pressure of 63 kg./cm.² sothat the reaction mixture formed in operation (2) below underwent anemulsion polymerization reaction as it passed through the reactor, thereactor residence time being about 30 minutes;

(2) during operation 1, constantly feeding to the reactor during eachhour (for each 100 parts of monomer) 400 parts of water containing 0.6parts of ammonium persulfate and 0.12 part sodium hydroxide andmaintaining the reaction mixture at a pH of 3.7;

(3) continuously removing from the reactor the resulting copolymer latexwhich was continuously formed during operations 1 and 2;

(4) after discarding the latex obtained during the first four residencetimes, collecting the desired quantity of latex and mixing it foruniformity, the latex having a pH of about 3.7 and a copolymer solidscontent of 19.2%; and

(5) isolating the resulting copolymer from the latex by the gradualaddition of a 4% aqueous solution of potassium aluminum sulfate untilthe copolymer is coagulated, washing the copolymer particles withdistilled water, removing the water by means of a filter apparatus, andthen drying the copolymer in a circulating air-over at 100° C. to amoisture content of less than 1%.

EXAMPLE 7

The product fluoroelastomer provided in step (a) of Example 6 wasvulcanized by mixing 100 parts of it with 20 parts of magnesium silicatepowder, 15 parts of magnesium oxide, 4 parts of triallyl isocyanurateand 3 parts of 2,5-dimethyl-2,5-ditertiarybutylperoxy hexane, and thenheating it in a press for 1/2 hour at 166° C., followed by 24 hours inan oven at 204° C.

Two samples of this vulcanized polymer were immersed in a solution of 11parts of K₂ S and 6.4 parts of sulfur in 300 parts of dimethylacetamidefor 1/2 and 4 hours respectively at 85° C. During this treatment, thesamples swelled and the surface turned black.

The treated samples were washed with distilled water and then immersedin chlorine water at 20° C. for 16 hours. Dyring this treatment, thesurface color changed to light yellow.

Finally, the samples were dried in a vacuum oven at 50° C. for 48 hours.

Examination of the wettability and surface resistivity of the samplesgave the results listed in Table I

                  TABLE I                                                         ______________________________________                                                      Surface Resistivity                                             Time in K.sub.2 S Sol. (h)                                                                  (ohm cm)       Wettability                                      ______________________________________                                        0             1.85 × 10.sup.13                                                                       Not Wettable                                     0.5           3.85 × 10.sup.8                                                                        --                                               4             4.35 × 10.sup.9                                                                        Wettable                                         ______________________________________                                    

Examination of the surface by ESCA showed increasing amounts of sulfuron the surface of the samples as treatment time increased.

                  TABLE II                                                        ______________________________________                                        Time in K.sub.2 S Sol. (h)                                                                     Sulfur Intensity (cps)                                       ______________________________________                                        0                 74                                                          0.5              171                                                          4                410                                                          ______________________________________                                    

EXAMPLE 8

    ______________________________________                                        (A)  A reactor was charged with                                                    TFE/BTFE 87/13 copolymer                                                      powder                 20 parts                                               Xylene                150 parts                                               Triisopropyl phosphite                                                                              13.5 parts                                              Di-t. butyl peroxide   2.0 parts                                         ______________________________________                                    

The charge was heated to 142° C. and held at that temperature for 7hours while the vapors which formed were removed by condensation.

The resulting product was removed from the liquid by filtration, washedwith xylene and then methanol, and dried at 100° C. It was found to havea phosphorus content of 1.16% and a water solubility of less that 1%.

(B) The product of (A), 7.88 parts dispersed in 40 parts of methanol,was mixed with 14.32 parts of a 55% aqueous dispersion of TFE/HFP 85/15copolymer.

(C) To a sparging flask were added

    ______________________________________                                        NaCl                630 parts                                                 Asbestos fibers     63 parts                                                   (Chlorbestos SP-25,                                                           Johns-Manville Co.)                                                          Distilled water    3087 parts                                                 ______________________________________                                    

This charge was then sparged with air for 11/2 hours.

(D) The product of (B) was added to the product of (C) and the mixturewas sparged with air for 30 minutes.

(E) A diaphragm was formed from the slurry of (D) directly on thecathode of a laboratory chlor-alkali cell by immersing the cathode inthe slurry and drawing the slurry on the cathode with vacuum accordingto the following schedule:

    ______________________________________                                        Start            10.2 mm of vacuum                                            After                                                                         1 minute         20.3 mm                                                      2 minutes        33  mm                                                       3 minutes        43.2 mm                                                      4 minutes        53.3 mm                                                      5 minutes        66  mm                                                       6 minutes        78.7 mm                                                      7 minutes        99.1 mm                                                      8 minutes        139.7 mm                                                     9 minutes        180.3 mm                                                     10 minutes       203.2 mm                                                     ______________________________________                                    

The cathode was then removed from the slurry, placed in a horizontalposition and dried by drawing a vacuum of 203.2 mm of Hg on the manifoldfor 10 minutes, followed by a vacuum of 508 mm for 20 minutes. Thecoated cathode was then held briefly at 100° C. under a vacuum of 508 mmand was then baked for 30 minutes at 275° C.

(f) the diaphragm-cathode produced in (E) was placed into a position ina laboratory chlor-alkali cell. Direct current was applied to theelectrodes. The cell required an average voltage of 3.189 to achieve acurrent efficiency of 95% as compared with a conventional asbestosdiaphragm, which in the same application required an average voltage of3.4-3.5.

Tetraethyl pyrophosphite and tetraethyl hypophosphate can be used in theforegoing procedure in place of triisopropyl phosphite, withsubstantially the same results.

I claim:
 1. In a process for the electrolytic production of chlorinefrom brine, the improvement comrpising keeping the chlorine produced atthe anode separated from the hydrogen and sodium hydroxide produced atthe cathode with a diaphragm comprising a fluoropolymer provided as ahydrophilic fluoropolymer containing nonterminal units represented bythe structure ##STR28## where R is an alkyl radical of 1-12 carbon atomsor a cycloalkyl radical of 3-12 carbon atoms,the hydrophilicfluoropolymer having a phosphorus content of about 0.1-10% by weight,not more than about 1% by weight of the fluoropolymer dissolving inwater at 20° C.